This invention relates to an improved method for preparing granular hydroxypropylated starch ethers having the property of cold-water dispersibility.
The preparation of highly-substituted starch and cellulose products by the use of reagents such as alkylene oxides or halohydrins under alkaline conditions is well known. However, the necessity for large quantities of these reagents and the strongly alkaline nature of the reaction media often results in destruction of the original granular structure of the starch along with concurrent problems of starch degradation, purification and recovery. While a low degree of substitution is easily achieved through reactions in aqueous media, the resulting starch lacks the necessary stabilization characteristics of the more highly substituted starches. Reactions in water-miscible solvents have been considered as a possible alternative, but are costly and offer little improvement over methods of the prior art.
To overcome these difficulties the use of "dry reactions" has been investigated. Such reactions are described in early German patents disclosing the treatment of dry commercial starches directly with a large excess of ethylene oxide under pressure, with starch-like or plastic-rubbery masses resulting therefrom. Large amounts of by-products are accumulated in working these processes, the disposal of which leads to considerable waste of reagents. A solution to this problem is disclosed in U.S. Pat. Nos. 2,516,634; 3,705,891; and 3,725,386 whereby neutral salts of inorganic acids are used as catalysts, hence eliminating the necessity for excess reagents and alkali. Typical catalysts employed in the processes of these patents include sodium chloride, sodium sulfate, calcium chloride, and water-soluble salts of phosphoric acid. While presenting an improvement over the prior art, the processes of these patents, nonetheless, suffer from some serious disadvantages. The use therein of water-soluble salts of mineral acids (such as hydrochloric, sulfuric, phosphoric, etc.) leads to formation of highly substituted starch fractions consisting of such undesirable and toxic by-products as chlorinated, sulfated, etc. hydroxypropyl compounds which must subsequently be removed in the purification procedure. The low solubility of these salts in solvents such as ethanol is a further disadvantage, leading to higher ash content and/or to an off-taste of the modified starches.
Accordingly, it is an object of this invention to provide a method of producing a superior hydroxypropylated starch ether retaining its original granular form and having improved cold-water dispersibility and no discernible off-taste.
Another object is to provide such a method capable of producing, in one embodiment thereof, a cold-water dispersible granular product which is essentially free of salts and the by-products therefrom and therefore suitable for human consumption.
A further object is to provide such a method capable of producing, in another embodiment thereof, a cold-water dispersible granular product useful as a thickener in industrial applications.
A still further object is to provide a method of producing a cold-water dispersible hydroxypropylated starch ether which reduces the toxicity and the quantity of the by-products.